The present invention relates to the field of multiphase flow measurement. The invention is illustrated in one example with regard to the measurement of multiphase flow from individual oil wells, but it will be recognized that the invention will have a wider range of applicability. Merely by way of example, the invention may be applied in the food processing industry, wet steam measurement, and others.
Industry utilizes or has proposed several methods to measure the production of individual oil wells. The conventional approach is to use a three-phase or two phase separator to separate the multi-phase fluid mixture into distinctive phases. In the case where a three-phase separator is employed, three separate outgoing streams (gas, free water, and an oil/water emulsion) are produced. Separate flow meters measure the respective flow rates of the outgoing streams of oil, water, and gas. An on-line "cut" meter determines the water content of the emulsion stream. The two-phase separator operates similarly to the three-phase separator except that the free water stream is omitted.
These test separators are relatively large in physical size, expensive to construct, and require an abundance of ancillary pressure control and flow regulating equipment. Accordingly, users of this approach do not provide the separators for an individual oil well. Instead, a single test separator services a group of wells. Each individual well is placed "on test" for a relatively short period of time, and its production is determined. After the well is removed from test, it is assumed that the production from the well does not vary substantially until the well is again placed on test.
Another approach involves measuring multiphase flow without the use of a separator. In U.S. Pat. No. 5,099,697, Agar uses two volumetric-type flow meters connected in series to measure multiphase flow. A flow restriction device between the flow meters produces a pressure drop between the meters. Combining the measurements of pressure drop between the two flow meters, the flow rates from the flow meters, and the phase fraction from a phase fraction meter, a flow computer calculates the respective flow rates of each phase components.
Another approach, such as that described by Northedge in U.S. Pat. No. 4,881,412, involves measuring the total flow rate of the multiphase fluid, taking a relatively small fluid sample from the bulk flow line and determining the phase fractions in the sample by various measurement means. This approach suffers the shortcomings of obtaining representative sample from the flow line and finding reliable on-line techniques to measure the phase fractions in the fluid sample.
Still another approach, such as that described in U.S. Pat. No. 4,951,700, involves using a small in-line gas separator to produce a gas stream and a liquid stream. The respective flow rates and liquid phase fractions are then measured. One major drawback of this approach is that the separator often does not provide adequate retention time for the entrained gas to be completely separated from the liquid phase. Measurement accuracy and equipment integrity in the liquid stream are greatly hampered by the gas-bearing liquid.
From the above it is seen that a continuous and accurate multi-phase flow measurement apparatus that is compact, low cost, reliable, and requires little maintenance is desired.